The present invention relates to a transparent touch panel substrate and a transparent touch panel employing the same.
Indium oxide containing from 3 to 10% by weight tin oxide (hereinafter referred to as xe2x80x9cITOxe2x80x9d) for use as a transparent electrode in liquid-crystal displays is widely used as a transparent electrode formed on a transparent substrate for transparent touch panels, because it is transparent and can be easily processed in electrode formation. Transparent conductive tin oxide films also are used as transparent electrodes for transparent touch panels because they have satisfactory transparency.
The composition of a transparent conductive ITO film for use in liquid-crystal displays is designed so that the film has the lowest possible value of electrical resistivity (specific resistance) Because of this, the conventional transparent conductive film has a problem that the resistance thereof is so low as will be described later that the film is unsuitable for use as electrodes in transparent touch panels, in which the electrodes are required to have moderate sheet resistance.
Attempts have been made to form a transparent conductive ITO film having a reduced thickness so as to impart moderate sheet resistance thereto. However, in order for a transparent conductive ITO film to have the necessary moderate sheet resistance, the thickness thereof should be reduced to about 10 nm or smaller. There has been a problem that such reduced thicknesses result in difficulties in obtaining values of sheet resistance with satisfactory reproducibility because of fluctuations of film deposition conditions, differences of heat history in a production process, etc.
Furthermore, the transmittance of light in substrates coated with a transparent conductive ITO film having a thickness in such a small-thickness range greatly depends on the film thickness as shown in FIG. 5 because of the refractive index of the film (about 2.0 at awavelength of 550 nm). Consequently, the substrates having this type of transparent conductive film have problems that it is difficult to stably obtain a given transmittance and that there is a large difference in transmittance between the uncoated areas of the transparent substrate and those areas of the transparent substrate which are coated with the transparent conductive film. Due to this difference in transmittance, products employing this conventional substrate have a drawback that the areas coated with the transparent electrode are easily recognized.
A technique for avoiding or overcoming such problems is known in which a high transmittance is obtained by regulating the thickness of an ITO film so as to result in an optical thickness (product of the refractive index n and the actual film thickness d; nd) of around 138 nm, which is the half-wave length (xc2xd)xcex (xcex=550 nm) of 550 nm, which is the central wavelength in the visible light region (see FIG. 5). However, the ITO film having such optical thickness equal to the half-wave length and hence attaining a high transmittance has a reduced value of resistance because of the inadequate specific resistance of the film. Consequently, the transparent touch panel produced with this technique has a problem that the amount of current flowing through the panel in its ON state is large, resulting in increased power consumption.
On the other hand, in the case of using tin oxide as a transparent electrode, a moderate degree of sheet resistance required of transparent electrodes for touch panels is obtained since tin oxide has a higher specific resistance than ITO films. However, there has been a problem that it is extremely difficult to process a tin oxide film in electrode formation therefrom and, in particular, it is difficult to use the tin oxide film to produce a touch panel having the function of delicate touch switching.
The invention is intended to overcome the above-described problems of the ITO film and tin oxide film for use as a transparent conductive film for a touch panel substrate.
Accordingly, an object of the invention is to provide a transparent conductive film having both of the following properties:
1) to have a high transmittance; and
2) to be capable of being easily processed in electrode formation therefrom.
Another object of the invention is to provide a substrate having a transparent conductive film which also has:
3) a value of sheet resistance suitable for obtaining a power-saving type touch panel.
The invention provides a transparent touch panel substrate comprising a transparent substrate and deposited thereon a transparent conductive metal oxide film which contains zinc, indium, and tin as metallic elements and is soluble in acids.
The transparent conductive film according to the invention is a film of a metal oxide mixture comprising zinc oxide, indium oxide, and tin oxide, which each has a refractive index of about 2.0 at a wavelength of 550 nm, or is constituted of a composite oxide comprising these metal oxides. Consequently, the transparent conductive metal oxide film according to the invention is characterized by having a refractive index of about 2.0 at a wavelength of 550 nm.
In the transparent conductive metal oxide film according to the invention, the zinc oxide is an ingredient which is soluble in acids and serves to regulate the specific resistance of the film. The indium oxide is an ingredient which is soluble in acids and serves in cooperation with tin to reduce the specific resistance of the film. The tin oxide is an ingredient which improves the wearing resistance of the film and improves resistance to chemicals including alkalis.
In a preferred embodiment of the touch panel substrate of the invention, the transparent conductive metal oxide film has a thickness of from 100 to 160 nm.
When a transparent conductive film according to the invention having the refractive index shown above is deposited on one side of a glass plate having a refractive index of about 1.5 at 550 nm, the transmittance of a light passing through both the transparent conductive film and the glass plate changes with the thickness of the film as shown in FIG. 5.
According to this preferred embodiment of the invention, since the transmittance is as high as about 90% as apparent from FIG. 5, there is no fear of giving a dark display. The optical thickness of this transparent conductive film roughly corresponds to a half of the design wavelength xcex. By regulating the thickness of the transparent conductive film to a value in the range of from 100 to 150 nm, the transmittance of a light passing through the transparent conductive film and the glass plate becomes closer to the transmittance for the glass plate alone (92%).
In another preferred embodiment of the touch panel substrate of the invention, the transparent conductive metal oxide film contains zinc in an amount of from 40 to 65 atomic % based on all metals and contains indium in an amount of from 0.25 to 1.3 times the amount of tin on an atomic basis.
Zinc contents exceeding 65 atomic % are undesirable in that the film has too high a specific resistance and that the alkali resistance of the film abruptly becomes poor with increasing zinc content. Furthermore, the metal oxide film having such too high a zinc content is undesirable in that the film, upon heating, e.g., in the step of laminating with a resin film, comes to have considerably increased electrical resistance and impaired heat resistance. On the other hand, zinc contents lower than 40 atomic % based on all metals are undesirable in that the film has too low a specific resistance.
Indium atom contents in the film exceeding 1.3 times the content of tin atoms are undesirable in that the film has not only a reduced specific resistance but reduced wearing resistance. Such too high indium contents are undesirable also from the standpoint of profitability because the cost of raw materials for the coating film is high. Furthermore, such too high indium contents are apt to result in difficulties in stably depositing a film having a given resistance on a substrate by sputtering. On the other hand, indium atom contents in the film lower than 0.25 times the content of tin atoms are undesirable in that the film has reduced solubility in acids and this makes it impossible to complete processing for electrode formation in a short time.
According to the embodiment described above, the transparent conductive film can be regulated so as to have a sheet resistance of about from 500 to 5,000xcexa9. Sheet resistances of the transparent conductive film lower than 500xcexa9 are undesirable in that when two-transparent substrates each coated with the transparent conductive film are disposed so that the conductive films face each other to fabricate a touch panel, then the amount of current flowing upon contact of the opposed transparent conductive films (in the ON state) is large, resulting in increased power consumption. On the other hand, sheet resistances of the transparent conductive film exceeding 5,000xcexa9 are undesirable in that the contact of the opposed conductive films results in unstable electrical connection.
Namely, according to the embodiment described above, the transparent conductive film not only can have a transmittance equal to that of the glass plate but also enables the production of a touch panel which is of the power-saving type and in which ON/OFF switching operations can be conducted without fail.
It is preferred to regulate the contents of zinc oxide, indium oxide, and tin oxide in the transparent conductive metal oxide film according to the invention so that the film has a specific resistance of from 5xc3x9710xe2x88x923 to 75xc3x9710xe2x88x923 xcexa9cm.
In still another embodiment of the touch panel substrate of the invention, the transparent substrate is a glass plate. Although this glass plate is not particularly limited in composition, a float glass having a soda-lime silicate composition (refractive index, 1.52) is generally preferred.
The invention further provides a resistive film type transparent touch panel employing the glass-based transparent touch panel substrate described above. This resistive film type transparent touch panel has satisfactory display quality because the transparent electrode has a high transmittance. Furthermore, by regulating the area of the transparent electrode to a given value, the touch panel can be of the power-saving type and switching operations can be conducted without fail.